1. Field of the Invention
In general, the invention relates to vacuum contactors emloying vacuum interrupters and in particular to the means used to provide for the overtravel of the contacts caused by the wear thereof during the normal operations of the vacuum contactor.
2. Description of the Prior Art
There are many designs of vacuum interrupters in existence. U.S. Pat. No. 4,002,867, issued Jan. 11, 1977 entitled "Vacuum Type Circuit Interrupters With a Condensing Shield at a Fixed Potential Relative to the Contact" is a representative example of such vacuum interrupters. An operating mechanism combined with one, two or three vacuum interrupters constitutes a vacuum contactor. In contradistinction to circuit breakers which are considered as principal protective devices during fault conditions in an electrical circuit and are designed for 20,000 to 50,000 operations, the vacuum contactor is used to start and stop various electric loads in response to signals generated by control devices such as push button switches, limit switches, and programmable controllers with the vacuum contactor being designed to have a lifetime of 2 to 3 million operations.
The main difference between vacuum contactors and conventional air break contactors is that the vacuum interrupters of the vacuum contactor break or interrupt the electric current inside a vacuum chamber instead of inside an air arc box. The vacuum chamber for the vacuum interrupter consists of a unit assembly of a sealed evacuated enclosure surrounding a fixed or stationary electrical contact and a moveable electrical contact. A portion of the moveable contact extends through a gas-tight metallic bellows which allows for the essentially linear motion of the moveable contact with respect to the stationary contact. The bellows is attached to the evacuated chamber by means of an end seal. Another end seal is provided for attaching the stationary contact to the enclosure. A ceramic sleeve or cylinder is provided to separate and electrically isolate the two contacts. The end seals are attached to the ends of the ceramic sleeve forming the evacuated chamber of the vacuum interrupter.
Because vacuum interrupters are normally closed by atmospheric pressure and an auxiliary contact spring, means must be provided to force the contacts into the open position which is the normal state for a deenergized contactor. The actual contact force holding the moveable and stationary contacts together inside each vacuum interrupter is the sum of the atmospheric force (atmospheric pressure times the mean area of the bellows) plus the force provided by the auxiliary contact spring and the mechanical spring force of the bellows. This auxiliary contact spring force increases the total force sufficiently to sustain closure of the contacts during high short circuit currents that tend to blow the contacts apart. In the deenergized condition, there is no electrical energy available to provide the force necessary to separate the contacts. Instead, one or more mechanical springs provide this contact opening force. In practice this spring, called the kickout spring, exerts sufficient force to maintain the contacts in the open position in a deenergized contactor. To close the contacts of the vacuum interrupter on command, an electromagnet is provided that when energized, will pull the operating mechanism closed, overcoming the force of the kickout spring and closing the contacts of the vacuum interrupter.
One inherent problem with typical vacuum contactors is the short travel of the vacuum contacts, for example, only 0.150 to 0.200 inches. This small travel is favorable for obtaining a high pulling force from the electromagnet, but causes difficulties in setting devices used to indicate contact wear. For a vacuum interrupter, the contact wear allowance or contact overtravel per pair of contacts in the interrupter may be 0.125 inches down to a minimum of only 0.020 inches, the measurement of which requires a feeler gauge rather than a mechanic's scale. The problem cannot be solved by increasing the travel of the moving contacts beyond 0.200 inches because the mechanical life of the metallic bellows decreases rapidly as the amount of contact travel increases. Furthermore, the actual contact faces inside the vacuum chamber cannot be observed directly, so this small measurement must be made on a secondary reference, usually an arm in the operating mechanism of the contactor through which a portion of the shaft of moveable contact extends.
One means presently used to set contact overtravel is a double nut arrangement. With the contacts closed, a large diameter hex nut is threaded onto the extended portion or shaft of the moveable contact that extends beyond the arm of the operating mechanism. By turning the large nut on the shaft of the moveable contact, an overtravel gap between a surface of the nut and the arm can be established. A second smaller diameter hex nut is then threaded onto the shaft to lock the first hex nut in place. Accuracy in this overtravel setting is important in that too small of a gap leads to premature replacement of the interrupter while too large of a setting can indicate that there is additional contact material remaining when in fact the contacts have actually eroded to the point of replacement. This latter condition can result in faulty closing or arcing in the contactor.
In most vacuum contactor designs space within which adjustments can be made is usually at a premium. In order to set the overtravel gap, the first hex nut must be restrained from turning while the second hex nut is tightened against it. Usually ribs on the arm or electrical barriers in the housing limit the angular travel of a common wrench and prevent the insertion of an adjustable wrench into the space available. With the hexagonal nuts and limited wrench clearance, an accurate setting of the overtravel gap is difficult to make. For example, if the threaded shaft of the contact has 18 threads to the inch then one turn amounts to a linear travel of 55 thousandths and one sixth turn (to accommodate the hex shape of the nut) can amount to 9 thousandths. If the desired setpoint is 120 thousandths (0.120), 9 thousandths represents 7.5% of the setpoint value. In addition, due to the fact that wrenches have finite widths for their bodies, the full angle for holding the first hex nut may not be possible leading to a setting discrepancy beyond 9 thousandths. Thus, a nut which can be locked in place without the use of a wrench allowing for a more accurate setting of the overtravel gap would be desirable.
Another problem encountered with the use of the hex nuts is the transmission of torque to the shaft of the moveable contact during tightening of the nuts. With some vacuum interrupters, application of torque to the moveable contact can damage the bellows or insulating seal of the interrupter leading to premature failure. Accordingly, nuts which can be tightened without transferring torque to the shaft of the contact would also be desirable.